[4.1.2 Backport] CBG-5759: add missing pauser pattern (test-only)

[torcolvin]

CBG-5759

Clean cherry pick of #8458 to 4.1.2. No conflicts; resulting diff is 2 files, +47/-7 — identical line counts to the upstream commit.

#8668 backported only the first CBG-5509 PR (#8414), which added the migrationPauser/compactionPauser helpers and applied them to most call sites. The follow-up #8458 applied the pauser to the two spots #8414 missed, and that half never came across, so 4.1.2 still flakes on them. Seen on #8635, where test (ubuntu) and test-disable-rev-cache both failed on the same assertion:

    attachment_migration_api_test.go:62:
        	Error:      	Not equal:
        	            	expected: 503
        	            	actual  : 200
        	Test:       	TestAttachmentMigrationAPI
        	Messages:   	Response status 200 "OK" (expected 503 "Service Unavailable")

The migration over the 10-doc dataset finishes ~1ms before the duplicate start request lands, so nothing is running and the second POST opens a fresh run with 200 instead of rejecting with 503.

• TestAttachmentMigrationAPI — adds LeakyBucketConfig, pauses at legacyKeys[0] and waits for the block before the duplicate-start 503 check.
• TestAttachmentMigrationMultiNode — pauser2 bound to rt2's own leaky datastore for the resumed run, plus a legacy doc created after Stopped is confirmed so the resumed run has something unmigrated to block on.
• TestAttachmentCompactionPersistence — same treatment for the compaction resume path.

Verified with -count=10 on TestAttachmentMigrationAPI, TestAttachmentMigrationMultiNode and TestAttachmentCompactionPersistence, plus both packages in full.

test-only — both changed files are _test.go.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR backports the “pauser” pattern into the remaining attachment migration/compaction API tests on the 4.1.2 branch to make the tests deterministic and eliminate flakes caused by background jobs completing before assertions observe the expected intermediate states.

Changes:

• Update TestAttachmentMigrationAPI to pause migration on a known legacy doc so a duplicate start reliably returns 503 Service Unavailable.
• Update TestAttachmentMigrationMultiNode and TestAttachmentCompactionPersistence to pause resumed runs on node 2 and ensure there’s always an unmigrated/unmarked doc to block on.
• Add additional assertions tying resumed runs to the original migration/compaction IDs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
rest/attachmentmigrationtest/attachment_migration_api_test.go	Adds pausing to reliably keep migration “running” during duplicate-start and resume-path assertions, including multi-node resume behavior.
rest/attachmentcompactiontest/attachment_compaction_api_test.go	Adds pausing and ID assertions to stabilize compaction resume-path expectations.

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